Computerized search engines are able to search for and return documents in response to receiving queries. The returned documents may come in any of a variety of forms, such as textual pages posted online (e.g., HTML pages, PDF pages, word processing documents, etc.), snippets of textual pages that are displayed as search results, search suggestions generated for an entered search request, and advertisements that may be displayed on a search results page. Search engines currently match and rank documents in search results based on a number of factors, such as popularity of the documents and how well the terms of documents match the query. There may be multiple rankings of response documents in response to a single query. For example, a search engine may rank the search result documents themselves, and the search engine may also rank advertisements to be displayed on a page of search results.